A known muffler provided for installation in the exhaust line of an internal combustion engine has an elongated doublewalled casing which is generally cylindrical and circular in cross section, its two wall parts contacting one another so as to be approximately free of play. The pulsating flows of gas contained in the exhaust flow are smoothed in the muffler so that the intake and exhaust noises of the internal combustion engine are deadened. This is achieved in that the exhaust flow is guided through at least one chamber and/or at least one perforated exhaust pipe enveloped by a sound absorbing material. In the method for producing such a muffler, two sheet metal pieces having a thickness of 0.5 mm are placed one upon the other and bent toward the longitudinal wall. During or after the bending, at least one exhaust pipe and the sound absorbing material are inserted into the casing. Following this, the edges of the two wall parts extending along a generating or surface line are turned up to form a longitudinal flange and are connected with one another at their ends and with a closing wall in each instance.
Since the two snugly contacting metallic wall parts have a relatively high sound and heat conductivity, this muffler has poor sound and heat insulation.
A second known muffler which differs only slightly from the muffler described above and is manufactured according to the same method comprises an intermediate space between the two wall parts. The wall parts are held at a distance from one another by a fiberglass layer located between them. The radial distance between the outer surface of the inner wall part and the inner surface of the outer wall part is normally approximately 1 mm to 2 mm.
The fiberglass layer improves the sound and heat insulation of the muffler. But this improvement is only slight, since the fiberglass layer used for improving the insulation is very thin and has a relatively high thermal conductivity. In addition to this, the fiberglass layer is relatively expensive.
Similar problems are also posed in the production of catalytic converters.
The object of the present invention is to develop a method by which a device forming a muffler or catalytic converter can be produced without the disadvantages of the described mufflers.